The present invention relates to an apparatus for determining the material level in a vessel. In particular, the present invention relates to a material level measurement apparatus which includes a signal generating unit, a coupling unit, a conductive element, and a receiving/evaluation unit.
A material level in a vessel is measured by means of measuring systems that measure different physical quantities. These quantities are then used to derive the desired information regarding the material level. In addition to mechanical sensors, capacitive, conductive, and hydrostatic measuring sensors, and sensors operating on the basis of ultrasonic, microwaves, or other electromagnetic radiation can be used to measure material level.
Many applications, for example, in the petrochemical, chemical, and food industries, require highly accurate measurements of the level of liquids or bulk materials in vessels (tanks, silos, etc.). Increasingly, sensors are used in which short high frequency electromagnetic pulses or continuous microwaves are coupled to a conductive cable sensor and by means of this cable sensor are introduced into the vessel in which the material is stored. This cable sensor can be any type of conductive element.
Physically, this measuring method utilizes the effect occurring at the interface between two different media, e.g. air and oil or air and water. A portion of the guided high frequency pulses or the guided microwaves is reflected at the media interface due to the abrupt change (discontinuity) of the dielectric constants of the two media. The reflected portion is returned via the conductive element to a receiving device. The reflected portion is greater for greater differences between the dielectric constants of the two media. The distance to the interface can then be calculated from the propagation time of the reflected portion of the high frequency pulses or microwaves. Knowing the empty distance of the vessel permits calculation of the material level within the vessel.
Sensors with guided high frequency signals (pulses or waves) are distinguished by significantly lower attenuation compared to sensors that freely emit high-frequency pulses or waves (free-field microwave systems (FMR) or  greater than true radar systems=). The reason is that the energy flow is highly concentrated along the cable sensor or the conductive element. Furthermore, sensors with guided high frequency signals provide higher measurement quality at close range than freely emitting sensors.
A further advantage of sensors with guided high frequency signals is the superior safety and reliability of the level measurement. This is due to the fact that measurement with guided measuring signals is largely independent of the product characteristics of the material (moisture, dielectric constant, material change), the vessel design (materials, geometry, or other operating conditions (dust, deposits, and reflection angle).
An objective of the present invention is to provide a sensor apparatus with a conductive element in which the conductive element of the sensor does not come into direct contact with the material being measured.
The present invention attains this objective in the following manner. A signal generating unit generates high frequency measuring signals. A coupling unit couples these measuring signals to a conductive element, the length of the conductive element corresponding at least to the maximum level of the vessel. The conductive element is disposed at a predetermined distance to the material in a vessel, the distance being such that the electromagnetic field produced by the measuring signals interacts with the material and is partially reflected when it meets the surface of the material in the vessel. The time characteristic of the reflected echo signals guided along the conductive element are detected by the receiving/evaluation unit and analyzed.
An alternative embodiment of the present invention provides that the conductive element is disposed outside the vessel. In this case, the vessel itself must be made of a non-conductive material at least within a defined sphere of influence of the conductive element. Materials that are typically used are plastics or glass.
The arrangement of the level measuring device outside the vessel has of course a number of obvious advantages. In addition to ensuring simple installation, the arrangement of the measuring device outside the material being measured prevents contamination or, in case of an aggressive material, corrosion of the conductive element. This makes it possible to use inexpensive materials to manufacture the conductive element. Costly encapsulation of the electrical parts of the measuring device may be largely eliminated.
In another embodiment of the present invention, the vessel, or at least a portion of the vessel within the sphere of influence of the electromagnetic field, is structured as a viewing glass which does not interfere with the electromagnetic field, and which can be transparent in the visible part of the spectrum. If the material level is measured directly on a vessel designed as a viewing glass, the invention can also include a conductive shield as an additional element. The shield is located on the opposite side of the viewing glass from the conductive element to ensure that interfering radiation is effectively shielded.
In another embodiment of the present invention, the conductive element is mounted directly on the vessel or the viewing glass by a simple connection. This connection is preferably an adhesive bond in which the conductive element is glued to the outside wall of the vessel or the viewing glass.
Furthermore, a protective shield made of an electrically conductive material is provided either in combination with the above embodiment or alone. The protective shield is positioned such that the conductive element is disposed between the vessel and the protective shield. The protective shield and the conductive element are spatially separate from each other. Preferably, a dielectric material is arranged between the protective shield and the conductive element. In the simplest case, this dielectric material is air. The protective shield can furthermore be constructed in such a way that it almost completely encloses the conductive element on the side facing away from the vessel. The purpose of the protective shield is to provide protection from interfering radiation from the space lying behind the conductive element, which can negatively affect the measuring accuracy of the level measurements.
The conductive element itself can have any shape as viewed in cross-section. It can for instance be circular, semi-circular, or polygonal. It is preferable that the conductive element be made of at least two conductors, whereby at least one of the two conductors is connected to ground. Improved measuring results are achieved particularly in the case where the conductive element is mounted outside the vessel and is moreover shielded from interfering radiation from the exterior by a protective shield. The reason is that in a multi-part embodiment of the conductive element, the electromagnetic field extending into the vessel is less strongly influenced by the protective shield.
While the above-described embodiments of the present invention describe the arrangement of the conductive element on the outside of the vessel, an alternative embodiment described below relates to an arrangement of the conductive element inside the vessel. In particular, a dielectric sheath or sleeve surrounds the conductive element at least in the area up to the maximum level of material in the vessel. Furthermore, the distance of the conductive element from the material to be detected is dimensioned in such a way that the measuring signal interacts with the material and is partially reflected when it strikes the surface of the material.
A desired distance between the conductive element and the material may be realized either through the selection of the thickness of the dielectric material and/or through the distance of the material from the conductive element located within the dielectric material. Furthermore, the conductive element may be disposed inside a sleeve made of a non-conductive material. The sleeve is placed inside the vessel and the conductive element is subsequently introduced into the sleeve. However, the conductive element and the sleeve may also be designed as a single unit that is then introduced into the vessel. For example, the conductive element may be surrounded directly by the non-conductive material.